Optical fiber is the major transmission medium in high capacity long distance communication systems. However, in short range data communication applications, such as local area networks, and in integrated applications, such as within the elements of a computer, automobile, watercraft, aircraft, satellite, machine or other similar systems, optical fiber has found only limited use. Such applications have been limited for several reasons. Among these are the higher costs of fiber systems relative to copper wire systems, and the difficulty and high cost of making fiber connections. A typical glass optical fiber has a relatively small core diameter, and a numerical aperture which makes connection or splicing of such fiber labor-intensive, time consuming, and expensive. Interest has centered on large core diameter optical fiber for local area network applications because of its ease of connection and splicing. However, typical large diameter fibers commercially available, are usually of the step indexed type, which have a bandwidth comparable to copper wire media. This bandwidth is generally too low for high data rate applications.
A step index optical fiber is characterized by a core having a uniform refractive index, surrounded by a cladding layer having a lower refractive index. FIG. 1 shows the refractive index profile for such a fiber. In a step index fiber, light is confined by total internal reflection between the core-cladding boundary. The confinement is best described by guiding modes, which can be visualized as light traveling at different angles with respect to the longitudinal axial direction. Modes traveling at larger angles travel further distances, and, hence, arrive at the fiber output end at different times. This phenomenon, termed intermodal time dispersion, or simply modal dispersion, limits the amount of optical signals that can be transmitted over the fiber; i.e., the bandwidth. For a typical large core step-index glass optical fiber, the bandwidth is limited to approximately 10 MHZ.multidot.km.
A possible solution to the modal dispersion problem is graded index optical fiber. It has been found that by continuously varying the refractive index from a maximum at the center of the optical fiber to a lower value at the core-cladding boundary, dispersion can be greatly reduced. Gloge and Marcatili, "Multimode Theory of Graded-Core Fibers," Bell System Technical Journal, November 1973, pp. 1563-1578, have proposed a graded index profile n(r) for a waveguide of radius r, given by: ##EQU1## wherein: n.sub.f is the refractive index at the center of the core; n.sub.c is the refractive index of the cladding layer; .DELTA.=(n.sub.f -n.sub.c)/n.sub.f ; and a is the core radius.
FIG. 2 shows such a graded index profile that varies continuously from the highest index value at the core center to a lower value at the core-cladding boundary. The waveguide bandwidth is increased because those modes traveling greater distances extend to greater radius and therefore travel faster due to the smaller average refractive index at greater radius. Such profiles can potentially reduce the delay time dispersion, and correspondingly, increase the bandwidth. For example, a graded index fiber having comparable core and cladding refractive indices as the step index fiber described above has a bandwidth of approximately 1-10 GHz.multidot.Km. However, graded index fibers have generally small numerical apertures, are difficult to fabricate, and are costly to manufacture.
Accordingly, there exists a need for optical waveguides, especially optical fibers, that combine both relatively high bandwidth and relatively high numerical aperture, and that can be fabricated using existing technology. This invention is directed to this important end.